This invention relates to high temperature, semi-crystalline, semi-aromatic nylon copolymers with improved shrinkage and blistering resistance, made from terephthalic acid (TA), hexamethylene diamine (HMD) and another aliphatic dicarboxylic acid as monomers. For many automotive or electrical/electronic applications, often rather complex parts are injection molded and dimensional stability of the moulded parts, under end-use conditions, becomes a critical attribute of performance. In these applications, post-molding performance or properties are strongly influenced by the level of crystallinity in a part, by the level of residual stresses in the polymer's amorphous phase and its moisture level. This is especially true for electronic/electrical (E/E) connector applications, where the cross-section of parts is rather thin, and where the same parts can be relatively long. Furthermore, these parts are molded under water-tempered mould conditions (i.e. .ltoreq.100.degree. C. mould surface temperature) and often stored under humid conditions. For electronic connectors, component integrity and retention of pin position requires excellent dimensional stability under the high temperatures encountered during the soldering process. In many instances, when electronic connectors, made of these polyamides, are soldered to circuit boards using the Infrared Solder Reflow technique, high surface temperatures encountered with this process result in blistering of the plastic connector.
Many copolymers based on TA and HMD described in the patent literature or in commercial existence give very marginal performance in these applications. Such is the case for 6T/DT [50/50 molar ratio], the copolymer of hexamethylene terephthalamide (6T) and 2-methyl pentamethylene terephthalamide (DT) (U.S. Pat. No. 5,378,800 to Mok, Pagilagan; U.S. Pat. No. 5,322,923 to Lahary, Coquard) because of its high glass transition temperature (Tg) and high post-molding shrinkage. A terpolymer of hexamethylene terephthalamide/hexamethylene isophthalamide/hexamethylene adipamide (6T/6I/66) [65/25/10 molar ratio], based on 3 acids. TA, isophthalic acid (1A) and adipic acid (AA), and HMD (U.S. Pat. Re. 34,447 to Poppe, Lisle, et al.) has high mold and post molding shrinkages when molded in cold molds. Similar would be the situation for the hexamethylene terephthalamide/hexamethylene isophthalamide/hexamethylene sebacamide (6T/6I/610) terpolymer [70/20/10 molar ratio] (U.S. Pat. No. 5,424,104 to Amimoto et al.) because of its high Tg. Such would also be the case of the copolymer of hexamethyleneterephthalamide/caprolactam (6T/6) [70/30] (EP 413258, to Kopietz, Betz, Blinne, Koch) because of its high T.sub.g. Copolymers of hexamethylene terephthalamide/hexamethylene adipamide (6T/66), based on TA, AA and HMD, at 55/45 molar ratio (U.S. Pat. No. 5,424,104) or 65/35 molar ratio (U.S. Pat. Re. 34,447) are functional in these "cold mold" injection molded parts, and display acceptable post mold shrinkage, but suffer in melt processability (i.e. thermal stability and flow), moisture adsorption and blistering resistance.
Polymers containing TA and HMD are generically described in the patent literature. Typical of that prior art is U.S. Pat. No. 5,200,450 to Blinne, Goetz, Hurley, Gutsche, Muehlbach (BASF) wherein the semi-aromatic polyamide is composed of I) 20-90% (wt) of units derived from terephthalic acid and hexamethylene diamine, ii) 0-50% (wt) of units derived from .epsilon.-caprolactam, iii) 0-80% (wt) units derived from adipic acid and hexamethylene diamine, and iv) 0-40% (wt) of further polyamide-dorming monomers. Also typical of that prior art is U.S. Pat. No. 5,324,766 to Ikejiri, Yamamoto, Kawamoto (Mitsui Petrochemicals Company), where the semi-aromatic polyamide resin consists essentially of i) a dicarboxylic acid component recurring unit consisting of 30-100 mole % of terephthalic acid component unit and 0-40 mole % of an aromatic dicarboxylic acid component unit other than terephthalic acid and/or 0-70 mole % on an aliphatic dicarboxylic acid component unit having an alkylene group of 4-20 carbon atoms and ii) a diamine component recurring unit comprising an alkylene diamine component unit having an alkylene group of 4 to 25 carbon atoms. The prior art though fails to provide specific examples of copolymers based on (1) TA and an aliphatic dicarboxylic acid other than AA, as the 2 "acid" monomers, in conjunction with (2) HMD as the diamine. Furthermore, the prior art cited fails to teach the molecular structure criteria by which to choose monomer combinations which deliver low T.sub.g, good dimensional stability under end-use conditions while at the same time maintaining high temperature performance and superior blistering resistance. In addition, none of the copolymers in commercial existence are based on HMD and 2 diacids, one being TA, the other, an aliphatic dicarboxylic acid other than AA, as briefly discussed above. The copolymers of the invention are based on TA and another aliphatic dicarboxylic acid, with HMD. These offer low post-molding shrinkage, good melt stability and processability, low moisture adsorption and excellent blistering resistance under SMT (surface mount technology) IR melt reflow soldering conditions required for electronic connectors.